Actuators are capable of changing form or shape in response to a stimulus or condition and, thus, to effect a transformation or action. Actuation is accomplished by, motors, piezoelectric devices, ion-exchange resins, and shape memory alloys, as examples. There is a need for reducing the weight of actuators, for reducing the noise associated with actuators, for reducing the currents and voltages required for their operation, and for increasing the strains that can be achieved. In general, actuators which are light-weight and make smooth motions are desired.
Recent advances in the actuator field relate to the use of polymers as constituents of actuator devices. Of particular interest are those polymeric systems that operate in an electromechanical mode; that is, which convert an electrical signal to a mechanical motion. One class of materials used for actuation is the family of conjugated polymers. Typical examples are polyaniline and polypyrrole. It was previously thought that in these materials volume change was a phenomenon that required mass transport. For example, under electrochemical stimulation, ions and solvents are exchanged between the conjugated polymer and its external environment. In the absence of such exchange, neither electrochemistry nor volume change occur. See, for example, “Electrochemical Studies Of Polyaniline And Its Application” by K. Okabayashi et al., Synth. Meth. 18, 365 (1987).
Electrochemical stimulation; that is, oxidation and reduction, of a conjugated polymer requires a counter electrode and an electrolyte. An electrolyte is either a liquid or a solid that contains ions. Oxidation and reduction of the conjugated polymer, which is the working electrode, is accompanied by other chemical reactions at the counter electrode. The working and counter electrodes are electrically insulated by the electrolyte, which is conductive through ion migration, but not electrically conductive.
Solid state versions of electrochemical actuators have been demonstrated in the laboratory. See, e.g., “Development Of An All Polymer Electromechanical Actuator” by T. W. Lewis et al., Polym. Prep. 38, 520 (1997), which teaches use of a gel electrolyte consisting of a mixture of propylene carbonate and ethylene carbonate solvents in polyacrylonitrile where in order to avoid loss of these solvents over time, the actuator would have to be sealed, and “A Solid State Artificial Muscle Based On Polypyrrole And A Solid Polymeric Electrolyte Working In Air” by J. M. Sansinena et al., Chem. Commun. 1997, page 2217 where a solid polymer electrolyte, poly(epichlorohydrin-co-ethylene oxide) is used. However, the muscle was found not function when the room-temperature humidity dropped below 60%. Furthermore, this type of device exhibited mechanical problems arising from poor adhesion between the conjugated polymer and the polymer or gel electrolyte. In these devices, a second conjugated polymer component usually acts as the counter electrode.
In “Electrically Induced Contraction Of Polypyrrole Film In Ambient Air” by H. Okuzaki and T. Kunugi, J. Polym. Sci. B 36, 1591 (1998), conjugated polymer actuators not based on electrochemical reactions are described. Polypyrrole was heated by driving a current though it which resulted in the loss of water molecules, causing the polypyrrole to contract. Upon cooling, the polypyrrole expanded as it reabsorbed water molecules from the environment.
Therefore, in order to achieve actuation using electrochemical conjugated polymer actuators present devices require a source of ions from a liquid electrolyte or from a solid polymer or gel electrolyte. This adds to the complexity of the device, and in order to avoid loss of water or solvent, the device must be sealed. For applications in certain environments, such as outer space, the requirement for water or solvents and the difficulty of sealing the device precludes their use. Conjugated polymer actuators based on driving water in and out of the polymer using heat generated by an electrical current, likewise cannot be used in low-humidity environments or for applications in outer space. Furthermore, the characteristics of such actuators, including the initial and final sizes as well as speed of response, are dependent on the ambient humidity.
Accordingly it is an object of the present invention to provide a conjugated polymer actuator responsive to electrical stimulation.
Another object of the present invention is to provide a conjugated polymer actuator responsive to electrical stimulation that does not require an electrolyte medium and a counter electrode.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.